fix(tracing): make CompositeScope.close idempotent and clear references

Author: jinseopkim0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/CompositeTracer.java

@@ -116,13 +116,17 @@ private static class CompositeScope implements Scope {
     public void close() {
       Throwable firstException = null;
       for (int i = count - 1; i >= 0; i--) {
-        try {
-          scopes[i].close();
-        } catch (Throwable t) {
-          if (firstException == null) {
-            firstException = t;
-          } else if (firstException != t) {
-            firstException.addSuppressed(t);
+        Scope scope = scopes[i];
+        if (scope != null) {
+          scopes[i] = null;
+          try {
+            scope.close();
+          } catch (Throwable t) {
+            if (firstException == null) {
+              firstException = t;
+            } else if (firstException != t) {
+              firstException.addSuppressed(t);
+            }
           }
         }
       }

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/tracing/CompositeTracerTest.java

@@ -35,6 +35,7 @@
 import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
@@ -125,6 +126,26 @@ void testInScope_childScopeCloseThrows() {
     inOrder.verify(scope1).close();
   }
 
+  @Test
+  void testInScope_compositeScopeClose_isIdempotent() {
+    ApiTracer.Scope scope1 =
+        mock(ApiTracer.Scope.class, Mockito.withSettings().withoutAnnotations());
+    ApiTracer.Scope scope2 =
+        mock(ApiTracer.Scope.class, Mockito.withSettings().withoutAnnotations());
+
+    when(child1.inScope()).thenReturn(scope1);
+    when(child2.inScope()).thenReturn(scope2);
+
+    ApiTracer.Scope compositeScope = compositeTracer.inScope();
+
+    compositeScope.close();
+    // Subsequent close should be idempotent and not invoke underlying scopes again
+    compositeScope.close();
+
+    verify(scope2, times(1)).close();
+    verify(scope1, times(1)).close();
+  }
+
   @Test
   void testOperationSucceeded() {
     compositeTracer.operationSucceeded();